The present invention relates to an image forming apparatus, such as an electrophotographic copying machine or a laser printer, and particularly to an improvement of a paper discharging mechanism of a fixing unit for fixing a toner image on a paper sheet.
In a conventional apparatus, as shown in FIG. 1, a paper sheet P is supplied through a vertical transfer path 103a from a developing unit (not shown) located below, and passes through a nip portion between a fixing roller 102 and a pressurizing roller 104, and further through a paper transfer guide 106 located above. The direction of the transfer is converted from the vertical to the horizontal by the paper transfer guide 106. Then, the paper sheet P is guided along a horizontal transfer path 103b to paper discharge driving rollers 110, and sent out on a tray (not shown) by the paper discharge driving rollers 110.
As shown in FIG. 2, the paper transfer guide 106 comprises a quarter cylinder having a curved surface in accordance with the transfer trail of the paper sheet P. A plurality of ribs 108 are attached on the circumferential surface of the quarter cylinder at regular pitch.
Normally, the fixing roller 102 and the paper discharge driving rollers 110 are drive-controlled, such that the circumferential velocity of the paper discharge driving rollers 110 is slightly higher than that of the fixing roller 102, in order to avoid jamming of the paper sheet P. Therefore, the paper sheet P may be rubbed against a diagonally shaded portion 108a of the ribs 108 shown in FIG. 1, in which case the toner image will be disordered.
Further, if a large amount of toner is fixed on the paper sheet P, the surplus toner is liable to adhere to and accumulate on the surface of the paper transfer guide 106 or the ribs 108. The adhering and accumulating toner may be adhered to a succeeding paper sheet P, and stain this paper sheet P.
In addition, since the distance from the fixing roller 102 to the paper discharge driving rollers 110 is short, the paper sheet P heated by the fixing roller 102 is discharged onto the tray through the paper discharge driving rollers 110 in a hot state. Therefore, when a user immediately picks up the paper sheet P from the tray, the user may feel pain due to the heat.
Furthermore, in the conventional apparatus, during a double-sided copying operation, the paper sheet P that has been discharged from the apparatus is returned to the inside of the apparatus. Therefore, the apparatus has the problem that the temperature in the apparatus rises due to the heat of the paper sheet P.
An object of the present invention is to provide an image forming apparatus that does not disorder a toner image or cause adhesion of toner on members in the paper transfer path and that can lower the temperature of the discharged paper sheet.
An image forming apparatus of the present invention comprises: a vertical transfer path through which a paper sheet having a toner image is transferred upward from below; a horizontal transfer path which is continuous to the vertical transfer path and through which the paper sheet is transferred in a substantially horizontal direction; a fixing roller which is brought into contact with one surface of the paper sheet on the vertical transfer path and which heats and melts toner of the toner image; a pressurizing roller which is opposite to the fixing roller and which is brought into contact with another surface of the paper sheet on the vertical transfer path and pressurizes the toner image in association with the fixing roller; a paper discharge driving roller which is provided at an end of the horizontal transfer path and which substantially horizontally discharges the paper sheet with the toner image fixed thereon; a tray which receives the paper sheet discharged from the paper sheet discharge roller; a paper transfer guide which is provided between the fixing roller and the paper discharge driving roller and which guides the paper sheet to be transferred along the vertical transfer path and the horizontal transfer path; and a paper discharge guide roller which is provided at a position between the vertical transfer path and the horizontal transfer path where a transfer direction of the paper sheet is changed from vertical to horizontal, and which rotates following motion of the paper sheet to be transferred, which is made of a high-conductivity material and which has a structure of a good heat radiating characteristic.
It is preferable that the paper discharge guide roller be covered with an antistatic resin film on its circumferential surface. Polytetrafluoroethylene is suitable for such a resin.
According to this embodiment, since the paper discharge guide roller is covered with a fluorinated ethylene resin-based film on its circumferential surface, toner does not adhere to the paper discharge guide roller. Particularly, even if the surface of a paper sheet, which is brought into close contact with the paper discharge guide roller, is a printed surface and a great amount of toner is fixed to the paper sheet, the toner will not adhere to the paper discharge guide roller.
Since the film covering the paper discharge guide roller may easily be charged, it is preferable that the film be conductive.
Polyacethylene, poly-p-phenylene and polypyrrole may be used as a conductive film. Static electricity charged on the paper sheet is discharged through the conductive film, so that electric coupling of the toner is released.
Preferably, the paper discharge guide roller comprises a metal cylinder having a hollow portion extending therethrough in an axial direction and has a plurality of inner ribs in the hollow portion. Straight blades, spiral blades or screw blades can be used as the inner ribs. In the case of spiral blades or screw blades, it is preferable that the blades be inclined 10xc2x0 to 45xc2x0 with respect to the axis of the paper discharge guide roller. When the paper discharge guide roller rotates following the paper sheet, the inner ribs shaped as the spiral blades or screw blades rotate to expel the heat from the hollow portion of the paper discharge guide roller, so that the paper discharge guide roller is cooled.
The hollow portion of the paper discharge guide roller may be exhausted by an electric fan.
Preferably, the image forming apparatus further comprises: an upper cover which is freely opened and closed to allow inspection of the paper discharge guide roller, the paper discharge driving roller, the paper transfer guide, the vertical transfer path and the horizontal transfer path, and which covers from above the paper discharge guide roller, the paper discharge driving roller, the paper transfer guide, the vertical transfer path and the horizontal transfer path when it is closed; a protective cover which is provided in non-contact with the paper discharge guide roller and made of material of a low-thermal-conductivity and which covers a circumferential surface of the paper discharge guide roller facing the horizontal transfer path, when the upper cover is opened; and a link mechanism which links to the protective cover and the upper cover such that the protective cover interlocks with the upper cover.
When the upper cover is opened, the circumferential surface of the paper discharge guide roller, which exposes in the horizontal path, is covered by the protective cover. Therefore, the operator can safely work without a possibility of a burn.
It is preferable that the image forming apparatus further comprise a peeling claw mechanism to peel the paper sheet from the paper discharge guide roller. Since it is ensured that the paper sheet is removed from the paper discharge guide roller by the peeling claw mechanism, paper-jamming trouble does not occur.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.